In a typical cellular radio environment, the signals between a base station and a subscriber terminal equipment propagate on several routes between a transmitter and a receiver. This multi-path propagation is mainly caused by signal reflections from surrounding surfaces. Signals travelling on different routes arrive at the receiver at different times because of a different propagation delay. This holds true for both directions of transmission. The multi-path propagation of a signal can be monitored in a receiver by measuring an impulse response of the received signal, signals arriving at different times being visible as peaks proportional to their signal strength. FIG. 1 illustrates a measured impulse response by way of example. Time is on a horizontal axis 100 and the strength of the received signals is on a vertical axis 102. Peaks 104, 106, 108 of the curve indicate the strongest multi-path propagated components of the received signal.
In prior art solutions the impulse response is estimated by means of a known training sequence added to the burst. FIG. 2 illustrates by way of example a normal burst of the GSM system comprising start and end bits 200, 202, actual data in two parts 204, 206 and a known training 208 sequence placed in the middle of the burst. In a normal burst, the length of the training sequence is 26 bits. In known solutions, such as in the GSM system, the impulse response is estimated by cross-correlating the received samples with a known training sequence. 16 bits of a 26-bit-long training sequence are used for estimating each impulse response tap. When the quality of the received signal is poor, it is difficult to attain a reliable estimation result with known methods. This will impair the performance of the receiver.